1. Field of the Invention
The present invention relates to an optical fiber drop cable suitable for a FTTH (Fiber to the Home) application, that is, an application where, in order to enable high speed wide band information such as ultra high speed data to be transmitted to and received from even a home or an office, an optical fiber cable extending from a telephone company allows an optical fiber cable core wire to be drawn and wired to a subscriber user such as a usual home and, more particularly, to an improvement over a tensile strength member forming an optical fiber drop cable (external wiring) to be used when drawing an optical fiber to a local home from an electric pole.
2. Description of the Related Art
Referring to FIG. 1A, an optical fiber drop cable 101 has a structure which is comprised of an optical fiber core wire 103 (or optical fiber tape-shaped core wire) with a diameter of, for instance, 0.25 mm and a pair of tensile strength members 105, 105, composed of conductive metallic wires each including a steel wire with a diameter of, for instance, 0.4 mm, which are disposed along the optical fiber core wire 103. The optical fiber core wire 103 and the adjacent tensile strength members 105, 105 are collectively covered with a cable sheath 107 made of thermoplastic resin, such as PVC or fire-retardant PE, to form an optical element section 109, which is integrally connected to a cable support wire section 115, serving as a tensile strength member, in parallel thereto via a constricted neck portion 117, with the cable support wire section 115 including a support wire 111 composed of a metallic wire, such as a steel wire with a diameter of, for instance, 1.2 mm, and covered with a thermoplastic resin sheath 113 such as PVC (polyvinyl chloride) and fire-retardant PE (polyethylene).
Also, as shown in FIG. 1A, the cable sheath 107 has both left and right sides formed with notch portions 119, 119 for cable splicing to take out the optical fiber core wire 103.
Further, heretofore known optical fiber drop cable of this type is disclosed in Japanese Patent Application laid open Publication No. 2001-83385. As shown in FIG. 1B, the fiber drop cable is comprised of an optical element 202 including an optical fiber core wire or an optical fiber tape-shaped core wire 201 and a pair of adjacent tensile strength members T, T composed of conductive metal wires, such as steel wires, respectively, with the optical fiber core wire 201 and the adjacent tensile strength members T, T being covered with a cable sheath S made of thermoplastic resin, and a cable support wire section 203 having a support wire R composed of metal, such as a steel wire, and integrally connected to the optical element section 202 in parallel thereto via a constricted neck portion 204. Further, the fiber drop cable is structured such that during installation, the support wire section 203 and the optical element section 202 are broken away from one another at the neck portion 205.
However, it is feared that not only the optical fiber drop cable 101 previously described but also the other related art optical fiber drop cable encounter accidents, such as burning out of indoor equipments, induced by lightening due to the presence of the conductive metallic wires forming the support wire 111 of the cable support wire section 115 and the tensile strength members 105, 105 of the optical element section 109. For this reason, attempts have heretofore been made for either an approach to cut the optical fiber drop cable 101 at the junction closure mounted to an indoor wall surface of the building or the local home to avoid the optical element section 109 from being drawn inside or an approach to cut only the tensile strength members 105, 105 (steal wires) inside the optical element section 109, using a specific tool, to allow the optical fiber core wire 103 to be drawn inside the home.
Incidentally, since the cable support wire section 115 is cut at and fixed to the electric pole or the eaves of the local home when drawing the cable support wire section 115 thereto, there is less problem.
In order to improve the deficiencies set forth above, by replacing the conductive metallic material forming the tensile strength members 105, 105 of the optical element section 109 in question with non-conductive material such as fiberglass, aramid fiber and FRP or the like to form a non-metallic configuration, the above issues may be improved.
Furthermore, with the usual optical fiber drop cable 101, if there is a low magnitude of adhesive force between the tensile strength members 105, 105 and the cable sheath 107, as explained in Japanese Patent Application laid open Publication No. 2000-171673, then the cable 101 undergoes bending and rigorous displacements or the like and, in addition, encounters thermal history such as heat cycles etc., resulting in an increase in optical transmission loss or troubles such as disconnections of the optical fiber core wire 103 encapsulated in the cable.
Accordingly, the adhesive force (extracting force) between the tensile strength members 105, 105 of the optical element section 109 and the cable sheath 107 forms an important factor in terms of the characteristics of the optical fiber drop cable. But, when manufacturing the above optical fiber drop cable 101 through extrusion forming, a mere technology of extrusion forming the con-conductive material, such as the fiberglass, aramid fiber, FRP (Fiber Reinforced Plastic) or the like, to be used as the tensile strength members 105, 105 inside the optical element section 109 together with the other members such as the support wire 111 and the optical fiver core wire 103 is hard to obtain an appropriate adhesive force. To increase the above adhesive force, with the invention disclosed in Japanese Patent Application laid open Publication No. 2000-171673, a layer of bonding agent is interposed between the tensile strength members 105, 105 and the cable sheath 107. However, the presence of the intervened bonding agent layer results in an increase in manufacturing costs.
The present invention has been made to address such issues mentioned above and has a first object to increase an adhesive force between tensile strength members inside an optical element section and a cable sheath without need for a layer of bonding agent. Further, the present invention provides an optical fiber drop cable, which enables an anti-twist turning property to be improved to avoid an increase in optical transmission loss.
It is a second object of the present invention to provide an optical fiber drop cable that has tensile strength members for an optical element section to be wired to an indoor area of a local home to be structured with non-conductive material of a low cost for avoiding a risk of being struck by lightening while enabling reduction in cost of the cable.
In order to achieve the above objects, an optical fiber drop cable according to a first aspect of the present invention comprises an elongated optical element section having an optical fiber core wire and at least one pair of first tensile strength members, disposed on both sides of the optical fiber core wire in parallel thereto to hold the optical fiber core wire between the first tensile strength members, which are covered with a cable sheath; and an elongated cable support wire section continuously or intermittently adhered to the elongated optical element section in parallel thereto and having a second tensile strength member covered with a sheath; wherein the first tensile strength members includes a collected body of tensile strength fibers made of non-conductive material.
According to the above first aspect, since the first tensile strength members of the optical element section are made of non-conductive material, it is possible to prevent an accident of being struck by lightening. Further more, according to the first aspect, the presence of intermittent connection between the optical element section and the cable support wire section provides an ease of separation between these components to provide an improved efficiency in installation.
An optical fiber drop cable according to a second aspect of the present invention concerns the optical fiber drop cable with the features of the above first aspect, wherein the non-conductive tensile strength fibers forming the first tensile strength members have a unit fiber with a Young""s modulus of a value not less than 5000 kg/mm2 and a collected amount of the fibers is in a range between 1000 denier and 2000 denier.
An optical fiber drop cable according to a third aspect of the present invention concerns the optical fiber drop cable with the features of the above first aspect, wherein a filling rate of fibers forming a collected body of the non-conductive tensile strength fibers forming the first tensile strength members is in a range above 50% of a cross sectional area of the collected body.
According to the above second and third aspects, there is less probable for the optical fiber to be disconnected when splicing the optical element section and the cable support wire section from one another with no increase in optical transmission loss of the optical fiber per se.
In order to achieve the above objects, an optical fiber drop cable according to a fourth aspect of the present invention comprises an elongated optical element section having an optical fiber core wire and at least one pair of first tensile strength members, disposed on both sides of the optical fiber core wire in parallel thereto to hold the optical fiber core wire between the first tensile strength members, which are covered with a cable sheath, and an elongated cable support wire section adhered to the elongated optical element section in parallel thereto and having a second tensile strength member covered with a sheath, wherein the first tensile strength members have outer peripheries formed in rugged configurations, respectively, to increase adhesive forces between the first tensile strength members and the cable sheath.
According to the optical fiber drop cable according to the fourth aspect of the invention set forth above, since the outer peripheries of the first strength members are formed in rugged configurations, increased adhesive force are created between the first tensile strength members and the cable sheath due to anchoring effects of the rugged configurations while providing an improved anti-twist turning characteristic and avoiding an increase in an optical transmission loss. Further, there is no need for coating a bonding agent as required in the related art practice during an extrusion forming process.
Further, an optical fiber drop cable according to a fifth aspect of the present invention concerns the optical fiber drop cable with the features of the above first aspect, wherein the first tensile strength members are made of non-conductive material.
According to the above fifth aspect, since the first tensile strength members of the optical element section are made of non-conductive material, it is possible to prevent an accident of being struck by lightening.
An optical fiber drop cable according to a sixth aspect of the present invention concerns the optical fiber drop cable comprises an elongated optical element section having an optical fiber core wire and at least one pair of first tensile strength members, disposed on both sides of the optical fiber core wire in parallel thereto to hold the optical fiber core wire between the first tensile strength members, which are covered with a cable sheath; and an elongated cable support wire section continuously or intermittently adhered to the elongated optical element section in parallel thereto and having a second tensile strength member covered with a sheath; wherein at least one of the first tensile strength members includes a collected body of tensile strength fibers made of non-conductive material.
According to the sixth aspect, since the first tensile strength members of the optical element section to be drawn into the local home are made of non-conductive material, it is possible to prevent a danger of being struck by lightening.
An optical fiber drop cable according to a seventh aspect of the present invention concerns the optical fiber drop cable with the features of the above first aspect, wherein the cable sheath covered on the optical fiber core wire and the first tensile strength members, and the sheath covered on the second tensile strength member are made of the same thermoplastic material and unitarily adhered to one another.
According to the above seventh aspect, since the above cable sheath and the sheath are integrally adhered to one another by means of the common thermoplastic resin, the cable sheath and the sheath can be collectively extruded to cover the optical fiber core wire, the first and second tensile strength members while permitting these components to travel together, with a resultant reduction in the number of manufacturing steps to enable fabrication in a single step.